Asymmetric performance multicore architecture with same instruction set architecture

ABSTRACT

A method is described that entails operating enabled cores of a multi-core processor such that both cores support respective software routines with a same instruction set, a first core being higher performance and consuming more power than a second core under a same set of applied supply voltage and operating frequency.

BACKGROUND

1. Field of Invention

The field of invention relates generally to computing systemarchitecture, and, more specifically, to an asymmetric performancemulticore architecture with same instruction set architecture (ISA).

2. Background

FIG. 1 shows a typical multi-core processor 100_1. As observed in FIG.1, the multi-core processor 100_1 includes a plurality of processorcores 101_1 to 101_N on a same semiconductor die 100_1. Each of theprocessor cores typically contain at least one caching layer for cachingdata and/or instructions. A switch fabric 102 interconnects theprocessor cores 101_1 to 101_N to one another and to one or moreadditional caching layers 103_1 to 103_N. According to one approach, theprocessors 101_1 to 101_N and the one or more caching layers haveinternal coherency logic to, for example, prevent two different coresfrom concurrently modifying the same item of data.

A system memory interface (which may also include additional coherencylogic) 104 is also included. Here, if a core requests a specific cacheline having a needed instruction or item of data, and, the cache line isnot found in any of the caching layers, the request is presented to thesystem memory interface 104. If the looked for cache line is not in thesystem memory 105_1 that is directly coupled to interface 104, therequest is forwarded through system network interface 106 to anothermulti-core processor to fetch the desired data/instruction from itslocal system memory (e.g., system memory 105_X of multi-core processor100_X). A packet switched network 107 exists between the multi-processorcores 100_1 to 100_X to support these kinds of system memory requests.

Interfaces to system I/O components 108_1 to 108_Y (e.g., deep nonvolatile storage such as a hard disk drive, printers, external networkinterfaces, etc.) are also included on the multi-processor core. Theseinterfaces may take the form of high speed link interfaces such as highspeed Ethernet interfaces and/or high speed PCIe interfaces.

Some multi core processors may also have a port 105 to the switch fabric102 to scale upwards the number of processor cores associated with asame (also scaled upward) caching structure. For example, as observedFIG. 1, multi-processor cores 101_1 and 101_2 are coupled through theswitch fabric port 105 to effectively form a platform of 2N cores thatshare a common caching structure (processor 100_2 is coupled toprocessor 100_1 through a similar port to its switch fabric).

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings, in which likereferences indicate similar elements and in which:

FIG. 1 shows a multicore processor and surrounding computer system(prior art);

FIG. 2 shows a power management strategy (prior art);

FIG. 3 shows a logic gate drive circuit;

FIG. 4 shows multi core processor having high power and low power coresthat support the same instruction set;

FIG. 5 compares power consumption of a high power core and low powercore;

FIG. 6 shows a first power management method;

FIG. 7 shows a second power management method;

FIG. 8 shows a design method.

DETAILED DESCRIPTION Overview

Detailed Description

Computing system power consumption is becoming more and more of aconcern. As such, a number of different power management schemes areincorporated into modern day computing systems. Typically, the powermanagement component of the system will scale up the processingperformance of the system as the system's workload increases, and, scaledown the processing performance of the system as the system's workloaddecreases. Decreasing the processing performance of the systemcorresponds to power savings as the power consumption of the system isstrongly correlated with its performance capabilities.

A typical way to scale processing performance and power consumption withworkload is to enable/disable entire cores and raise/lower their supplyvoltages and operating frequencies in response to system workload. Forexample, as observed in FIG. 2, under a maximum performance and powerconsumption state 201 all cores are enabled and each core is providedwith a maximum supply voltage and maximum clock frequency. By contrast,under a minimum performance and power consumption state 202 (at whichprogram code can still be executed), only one core is enabled. Thesingle core is provided with a minimum supply voltage and minimumoperating frequency.

Some basic concepts of electronic circuit power consumption are observedin FIG. 3. Here, the driver circuit 310 portion of a logic gate 311 isobserved driving a next one or more logic gate(s) 312. Specifically, thespeed of operation of interconnected logic gates 311, 312 rises as thewidth of its driving transistors 302_1, 302_2 (measured, for eachtransistor, along the semiconductor surface perpendicular to thedirection of current flow) increase and the capacitance 303 of the line304 (and input capacitance of the load logic gate(s) 312) it is drivingdecreases. Here, in order to raise the voltage on the line from a logiclow level to a logic high level, a sufficiently strong current 305 needsto be driven by the source transistor 302_1 through the line to rapidlyapply charge to the capacitance 303 (and thereby raise the voltage onthe line). Similarly, in order to lower the voltage on the line from alogic high level to a logic low level, a sufficiently strong current 306needs to be “sunk” by the sink transistor 302_2 through the line torapidly draw charge off the capacitance (and thereby lower the voltageon the line).

Essentially, the amount of current the transistors 302_1, 302_2 willsource/sink is a function of their respective widths. That is, the widerthe transistors are, the more current they will source/sink. Moreover,the amount of current the transistors 302_1, 302_2 will source/sink isalso a function of the supply voltage VCC that is applied to the drivercircuit 310 observed in FIG. 3. Essentially, the higher the supplyvoltage, the stronger the source/sink currents will be.

Further still, the rate at which the transistors will be able toapply/draw charge to/from the capacitor is a function of the size of thecapacitance 303 of the line 304 being driven. Specifically, thetransistors will apply/draw charge slower as the capacitance 304increases and apply/draw charge faster as the capacitance 304 decreases.The capacitance 304 of the line is based on its physical dimensions.That is, the capacitance 304 increases the longer and wider the line,and by contrast, the capacitance 304 decreases the shorter and narrowerthe line is. The line itself is of fixed dimensions once the circuit ismanufactured. Nevertheless, line width and line length are designparameters that designers must account for. The width of the line cannotbe narrowed too much or else it will have the effect of increasing theline's resistance which will also slow down the rate of chargeapplied/drawn to/from the capacitor.

A final speed factor is the frequency of the signal itself on the line.Essentially, circuits driven with a faster clock signal will morerapidly switch between applying and drawing charge to/from the linecapacitance 304 than circuits with a slower clock signal. Here, morerapid switching corresponds to a circuit that is sending binaryinformation faster.

All of the factors described above for increasing the rate at which thecharge on the capacitor is applied/drawn also lead to a circuit thatconsumes more power. That is, a circuit that is designed to haverelatively wide source/sink transistors, a high supply voltage, shortload lines and receive a higher frequency clock signal will operatefaster and therefore consume more power than circuits oppositelyoriented as to these same parameters.

Recalling the discussion of FIGS. 1 and 2, note that prior art multicore processor power management schemes have been implemented onprocessors whose constituent cores are identical. That is, referring toFIG. 1, all of cores 101_1 to 101_N are identical in design. In otherapproaches, the cores are not identical but are radically different.Specifically, one of the cores is a low power core but the lower powercharacteristic is achieved by stripping out sizable chunks of logiccircuitry as compared to the other cores. More specifically, the sizablechunks that are stripped out correspond to the logic that executes theprogram code instructions. Said another way, the low power core supportsa reduced instruction set as compared to the higher performance cores. Aproblem with this approach, however, is that it is difficult for systemsoftware to adjust switch operation between processor cores havingdifferent instruction sets.

FIG. 4 depicts a new approach in which at least one of the cores 401 isdesigned to be lower performance and therefore consume less power thanother cores 402 in the processer. However, the lower power core(s) 401has a same logic design as the higher power core(s) 402 and thereforesupports the same instruction set 403 as the high power core(s) 402. Thelow power core(s) 401 achieve a lower power design point by havingnarrower drive transistor widths than the higher power core(s) and/orhaving other power consumption related design features, such as any ofthose discussed above with respect to FIG. 3, that are oppositelyoriented than the same design features in the higher power cores.

According to one approach, discussed in more detail below, when themulti-processor core is being designed, the same high level description(e.g., the same VHDL or Verilog description) is used for both the higherperformance/power core(s) and the lower performance/power core(s). Whenthe higher level descriptions are synthesized into RTL netlists,however, for the subsequent synthesis from an RTL netlist into atransistor level netlist, different technology libraries are used forthe low power core(s) than the high power core(s). As alluded to above,the drive transistors of logic gates associated with the libraries usedfor the low power core(s) have narrower respective widths than the“same” transistors of the “same” logic gates associated with thelibraries used for the high power cores.

By design of the multiprocessor, referring to FIG. 5, the lower powercore(s) exhibit inherently lower power consumption (and processingperformance) than the higher power core(s). That is, for a same appliedclock or operating frequency, because of its narrower drive transistorwidths, for example, a lower power core will consume less power than ahigher power core. Because of the narrower drive transistor widths,however, the lower power core has a maximum operating frequency that isless than the maximum operating frequency of the higher power core.

The import of the lower power core, however, is that the multi-processoris able to entertain a power management strategy that is thesame/similar to already existing power management strategies, yet, stillachieve an even lower power consumption in the lower/lowestperformance/power states. Specifically, recall briefly power state 202of FIG. 2 in which only one core is left operable (the remaining coresare disabled). Here, if the one remaining operable core is the low powercore, the processor will exhibit even lower power consumption than theprior art low power state 202.

The amount of reduced power savings 503 is directly observable in FIG.5. Here, recall that all the processors were identical in themulti-processor that was discussed with respect to the prior art lowpower state 202 of FIG. 2. As such, even if the supply voltage andoperating voltage was reduced to a minimum, the power consumption wouldbe that of a higher power processor (e.g., having wider drive transistorwidths). This operating point is represented by point 504 of FIG. 5. Bycontrast, in the lowest power operating state of the improvedmulti-processor, if the operable core is a low power core it willconsume power represented by point 505 of FIG. 5. As such, the improvedprocessor exhibits comparatively lower power consumption at thelower/lowest performance operating states than the prior artmulti-processor, while, at the same time, fully supporting theinstruction set architecture the software is designed to operate on.

FIG. 6 shows a power management process flow that can be executed, forexample, with power management software that is running on themulti-processor (or another multi-processor or separate controller,etc.). Conversely, the power management process flow of FIG. 6 can beexecuted entirely in hardware on the multi-processor or by somecombination of such hardware and software.

According to the process flow of FIG. 6, from an initial state 601 whereat least some high power processor cores and the low power core(s) areoperating, in response to a continued drop in demand on themulti-processor, another high power core is disabled each time thecontinued drop in demand falls below some next lower threshold. Forexample, in a multi-processor core having sixteen cores where fourteencores are high power cores and two cores are low power cores, theinitial state 601 may correspond to a state where seven of the highpower cores and both of the low power cores are operational.

In response to continued lower demand placed on the multi-processor, theseven high power cores will be disabled one by one with each new lowerdemand threshold 602. For instance, as observed at inset 610, demandlevel 611 justifies enablement of the seven high power cores and bothlow power cores. As the demand continually drops to a next lowerthreshold 612, one of the high power cores is disabled 613 leaving sixoperable high power cores and two low power cores.

Before the high power core is disabled, as a matter of designer choice,the core's individual operating frequency, or the operating frequency ofall (or some of) the enabled high power cores, or the operatingfrequency of all (or some of) the enabled high power cores and the lowpower cores may be lowered to one or more lower operating frequencylevels.

A similar designer choice exists with respect to the supply voltagesapplied to the cores. That is, before the high power core is disabled,as a matter of designer choice, the core's individual supply voltage, orthe supply voltage of all (or some of) the enabled high power cores, orthe supply voltage of all (or some of) the enabled high power cores andthe low power cores may be lowered to one or more lower supply voltages.Supply voltages may be lowered in conjunction with the lowering ofoperating frequency, or, just one or none of these parameters may belowered as described above.

Eventually, with the continued drop in demand, the last remaining highpower core will be disabled 615 after demand falls below some lowerthreshold 614. This leaves only the low power cores in operation.Operating frequency and/or supply voltage of the low power core(s) maylikewise be lowered as demand continues to drop beneath level 614. Withcontinued drop in demand a similar process of disabling cores as demandfalls below each next lower demand threshold 604 continues until themulti-processor core is left with only one low power core remaining asits sole operating core 606.

State 606 is reached of course with the disablement of the last highpower core in implementations where the processor only has one lowerpower core. Again supply voltage and/or operating frequency of the soleremaining low power core may be lowered as demand continues to fall.Importantly, in state 606, as discussed above, the multi-processor willexhibit lower power consumption than other multi-processor cores havingan identical power management scheme but whose constituent cores are allhigh power cores. Even lower power consumption can be provided for instate 606 if the sole operating low power core is provided with a lowersupply voltage and/or lower operating frequency that the lowestoperating supply voltage and/or operating frequency applied to the highpower cores.

No special adjustment needs to be made by or for application software,virtual machine or virtual machine monitor when the system is runningonly on the low power core(s) after all the high power cores aredisabled. Again, the preservation of the same instruction core acrossall cores in the system corresponds to transparency from the software'sperspective as to the underlying cores. Lower performance may berecognized with lower cores but no special adjustments as to the contentof the instruction streams should be necessary. In various alternateimplementations: 1) the hardware/machine readable firmware can monitorand control the core mix; or, 2) the hardware can relinquish control tothe Operating system and let it monitor the demand and control the coremix.

FIG. 7 shows essentially a reverse of the processes described above. Asobserved in FIG. 7, starting from a state in which only a single lowpower core is operating 701 additional low power cores are enabled (ifany more) 702 as demand on the multi-processor continually increases.Eventually, high power cores are enabled 703. Notably, the demandthreshold needed to enable a next processor from an operating low powerprocessor may correspond to a lower demand increment than the demandthreshold needed to enable to a next processor from an operating highpower processor.

That is, inset 710 shows the increase in demand 711 needed after a lowpower processor is first enabled to trigger the enablement of a nextprocessor in the face of increased demand. The increase in demand 712needed after a high power processer is first enabled to triggerenablement of a next high power processer in the face of increaseddemand is greater than the aforementioned demand 711. This is so becausea high power processor is able to handle more total demand than a lowpower processor and therefore does not need to have additional “help” assoon as a low power processor does.

Operating frequency and/or supply voltage may also be increased inconjunction with the enablement of cores in the face of increased demandin a logically inverse manner to that discussed above with respect tothe disablement of cores.

FIG. 8 shows a design process for designing a multi-core processorconsistent with the principles discussed above. As part of the designprocess, high level behavioral descriptions 800 (e.g., VHDL or Verilogdescriptions) for each of the processor's cores are synthesized into aRegister Transfer Level (RTL) netlist 801. The RTL netlist issynthesized 802 into corresponding higher power core gate levelnetlist(s) (one for each high power ore) with libraries corresponding toa higher power/performance design (such as logic circuits having widerdrive transistors). The RTL netlist is also synthesized 803 intocorresponding lower power core gate level netlist(s) (one for each lowpower core) with libraries corresponding to a lower power/performancedesign (such as logic circuits having wider drive transistors). Here,the logic designs for the high power and low power cores are the samebut the design of their corresponding logic circuits have differentperformance/power design points.

The transistor level netlists for the respective cores are then used asa basis for performing a respective place, route and timing analysis 806and design layout 807. Here, the lower power/performance cores may havemore relaxed placement and timing guidelines owing to the largerpermissible propagation delay through and between logic circuits. Saidanother way, recalling from the discussion of FIG. 3 that longer loadlines result in slower rise and fall times, the lower performance coresmay permit longer load lines between transistors and gates because thesecores are designed to have slower operation (of course, if load linesare increased to much along with the inclusion of narrower drivetransistors, the drop in performance may be more than desired).

Upon completion of the layout and timing analysis, the cores are clearedfor manufacture upon a clean manufacturing ground rule check 808.

Processes taught by the discussion above may be performed with programcode such as machine-executable instructions that cause a machine thatexecutes these instructions to perform certain functions. In thiscontext, a “machine” may be a machine that converts intermediate form(or “abstract”) instructions into processor specific instructions (e.g.,an abstract execution environment such as a “virtual machine” (e.g., aJava Virtual Machine), an interpreter, a Common Language Runtime, ahigh-level language virtual machine, etc.)), and/or, electroniccircuitry disposed on a semiconductor chip (e.g., “logic circuitry”implemented with transistors) designed to execute instructions such as ageneral-purpose processor and/or a special-purpose processor. Processestaught by the discussion above may also be performed by (in thealternative to a machine or in combination with a machine) electroniccircuitry designed to perform the processes (or a portion thereof)without the execution of program code.

It is believed that processes taught by the discussion above may also bedescribed in source level program code in various object-orientated ornon-object-orientated computer programming languages (e.g., Java, C#,VB, Python, C, C++, J#, APL, Cobol, Fortran, Pascal, Pert, etc.)supported by various software development frameworks (e.g., MicrosoftCorporation's .NET, Mono, Java, Oracle Corporation's Fusion, etc.). Thesource level program code may be converted into an intermediate form ofprogram code (such as Java byte code, Microsoft Intermediate Language,etc.) that is understandable to an abstract execution environment (e.g.,a Java Virtual Machine, a Common Language Runtime, a high-level languagevirtual machine, an interpreter, etc.) or may be compiled directly intoobject code.

According to various approaches the abstract execution environment mayconvert the intermediate form program code into processor specific codeby, 1) compiling the intermediate form program code (e.g., at run-time(e.g., a JIT compiler)), 2) interpreting the intermediate form programcode, or 3) a combination of compiling the intermediate form programcode at run-time and interpreting the intermediate form program code.Abstract execution environments may run on various operating systems(such as UNIX, LINUX, Microsoft operating systems including the Windowsfamily, Apple Computers operating systems including MacOS X,Sun/Solaris, OS/2, Novell, etc.).

An article of manufacture may be used to store program code. An articleof manufacture that stores program code may be embodied as, but is notlimited to, one or more memories (e.g., one or more flash memories,random access memories (static, dynamic or other)), optical disks,CD-ROMs, DVD ROMs, EPROMs, EEPROMs, magnetic or optical cards or othertype of machine-readable media suitable for storing electronicinstructions. Program code may also be downloaded from a remote computer(e.g., a server) to a requesting computer (e.g., a client) by way ofdata signals embodied in a propagation medium (e.g., via a communicationlink (e.g., a network connection)).

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will, however,be evident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the appended claims.

1. A multi-core processor comprising a plurality of cores that support asame instruction set, where, at least one of the cores is constructed oflogic gates that consume less power, for a same applied operatingfrequency and supply voltage, than the same corresponding logic gates ofat least one other of the cores.
 2. The multi-core processor of claim 1wherein the logic gates that consume less power have narrower logic gatedriver transistors than the same corresponding logic gates of the atleast one other of the cores.
 3. The multi-core processor of claim 1wherein there are at least two cores that are constructed of the logicgates that consume less power.
 4. The multi-core processor of claim 1wherein only one of the cores is constructed of the logic gates thatconsume less power.
 5. The multi-core processor of claim 1 furthercomprising a switch fabric between the cores and a system memoryinterface.
 6. A method, comprising: operating enabled cores of amulti-core processor such that both cores support respective softwareroutines with a same instruction set, a first core being higherperformance and consuming more power than a second core under a same setof applied supply voltage and operating frequency.
 7. The method ofclaim 6 further comprising: disabling the first core but leaving thesecond core operable in response to lower demand being offered to themulti-core processor.
 8. The method of claim 7 further comprisingenabling the first core after it is disabled in response to higherdemand being offered to the multi-core processor.
 9. The method of claim8 wherein said method further comprises: disabling said first core afterdetecting a first drop in demand offered to said multi-core processor;enabling said first core after detecting a first rise in demand offeredto said multi-core processor, the amount of demand associated with saidfirst drop being greater than the amount of demand associated with saidsecond drop.
 10. The method of claim 6 wherein the operating of thefirst core includes driving load lines with wider transistor widths thancorresponding transistor widths in said second core.
 11. The method ofclaim 6 further comprising lowering a supply voltage and/or operatingfrequency of said first core in response to lower demand being offeredto said multi-core processor.
 12. The method of claim 6 furthercomprising raising a supply voltage and/or operating frequency of saidfirst core in response to higher demand being offered to said multi-coreprocessor.
 13. The method of claim 6 further comprising: disabling saidfirst core in response to a drop in demand applied to said multi-coreprocessor; and, lowering a supply voltage and/or operating frequency ofsaid second core in response to an additional drop in demand applied tosaid multi-core processor.
 14. A machine readable medium containingprogram code that when processed by a machine causes a method to beperformed, the method comprising: operating enabled cores of amulti-core processor such that both cores support respective softwareroutines with a same instruction set, a first core being higherperformance and consuming more power than a second core under a same setof applied supply voltage and operating frequency; disabling the firstcore but leaving the second core operable in response to lower demandbeing offered to the multi-core processor;
 15. The machine readablemedium of claim 14 wherein said method further comprises enabling thefirst core after it is disabled in response to higher demand beingoffered to the multi-core processor.
 16. The machine readable medium ofclaim 15 wherein said method further comprises: disabling said firstcore after detecting a first drop in demand offered to said multi-coreprocessor; enabling said first core after detecting a first rise indemand offered to said multi-core processor, the amount of demandassociated with said first drop being greater than the amount of demandassociated with said second drop.
 17. The machine readable medium ofclaim 14 wherein said method further comprises lowering a supply voltageand/or operating frequency of said first core in response to lowerdemand being offered to said multi-core processor.
 18. The machinereadable medium of claim 14 wherein said method further comprisesraising a supply voltage and/or operating frequency of said first corein response to higher demand being offered to said multi-core processor.19. The machine readable medium of claim 14 wherein said method furthercomprises: disabling said first core in response to a drop in demandapplied to said multi-core processor; and, lowering a supply voltageand/or operating frequency of said second core in response to anadditional drop in demand applied to said multi-core processor.